As concern for protection of the environment increases, regulations for preventing the escape to the atmosphere of volatile hydrocarbon fuel vapors are being promulgated by governmental agencies. One source of these hydrocarbon vapors is the fuel tanks of vehicles using gasoline or other hydrocarbon fuels of high volatility. With present automotive gas tank designs, fuel vapor can escape during the filling of the tanks and usually even after the tanks are filled. The use of an onboard vapor recovery system to remove excess fuel vapor from the fuel tank is one solution to the problem. Some vapor recovery systems utilize a vapor vent valve located near the top of the fuel tank through which fuel vapor flows to a vapor canister downstream of the vent valve when it is open. Typically, the vapor vent valve is responsive to the level of fuel in the tank and when a desired maximum level of fuel in the tank is reached during filling of the tank, the vapor vent valve is closed thereby increasing the pressure within the fuel tank to actuate an automatic shutoff device in the refueling nozzle.
The fuel dispensing nozzle of a typical station fuel pump has an automatic shutoff which is actuated by liquid fuel rising sufficiently in the fill tube of the fuel tank to at least momentarily cover and close off a control port immediately adjacent the outlet end of the dispensing nozzle. Closing this control port causes a vacuum actuated diaphragm and valve to automatically shut off the discharge of liquid fuel from the dispensing nozzle. During refueling of the fuel tank, there is the potential danger of over-pressurizing and even rupturing the fuel tank. Of particular concern is the failure of the automatic shutoff device of a fuel pump refueling nozzle to activate despite increasing pressure within the fuel tank. Most refueling pumps can supply fuel at a pressure that is more than enough to rupture the fuel tank and thus, the fuel tank can be over pressurized and damaged if the automatic shutoff device fails to operate.
Further, some filler neck designs for vehicle fuel tanks do not achieve a liquid seal during filling, especially at low fuel flow rates. If a sufficient liquid seal is not present during the refueling process, fuel vapors can recirculate back up through the filler neck and escape into the atmosphere. In addition, some refueling nozzles are designed to provide a vapor tight seal at the filler neck inlet during refueling. Typically, these nozzles also generate a vacuum or negative pressure to draw fuel vapors therein. If the vapor recovery system is not properly designed, the vacuum generated can draw liquid fuel back up the filler neck to the refueling nozzle which may then activate the automatic shutoff and prematurely terminate refueling.